1. Field of the Invention
The present invention relates to a method of electron-beam exposure that is mainly employed to manufacture a semiconductor device, and a mask as well as an electron-beam exposure system used therein, and more particularly to a mask for electron-beam exposure that is especially suited for the proximity effect correction in the scattering-angle limiting type electron-beam exposure method.
2. Description of the Related Art
In the electron-beam exposure, the proximity effect that may be caused by scattered electrons in a resist layer and within a substrate strongly affects the linewidth accuracy of patterns, which makes the proximity effect correction one of the essential techniques in the art.
In the cell projection lithography which is the most widely used method of electron-beam exposure, in order to obtain the amount of the correction dose, the dose compensation method which requires complicated calculations by the self-consistent method using the exposure intensity distribution (EID) function or the pattern density method has been currently used.
Meanwhile, in the scattering-angle limiting type (referred to as xe2x80x9cSAL typexe2x80x9d, hereinafter) electron-beam exposure method which has been, in recent years, attracting much interest as a next-generation electron-beam exposure technique, the correction of the proximity effect is conducted by a compensation method based on the GHOST method. The SAL type electron-beam exposure method employs the segmented transcription method in which patterns for the whole chip to be exposed. are set in a mask and, through scanning this mask, patterns are transcribed to a wafer. An exposure system used in this electron-beam exposure method is described in a later section, together with the GHOST method, in detail.
With respect to a mask for this SAL type electron-beam exposure method, there is used a mask (referred to as a xe2x80x9cscattering membrane maskxe2x80x9d, hereinafter) in which patterns made of an electron-beam scatterer, for example, tungsten with a thickness of 50 nm or so, is formed on an electron-beam transmittable membrane (referred to simply as a xe2x80x9cmembranexe2x80x9d, hereinafter) with a relatively small electron-beam scattering power, for example, a silicon nitride film with a thickness of 100 nm or so. The exposure is carried out by an electron beam consisting of electrons which are not scattered or scattered only with relatively small scattering angles, having transmitted the membrane, and the image contrast is formed on the wafer due to the difference of the electron-beam scattering between the membrane region and the scatterer region.
In the SAL type electron-beam exposure method, the proximity effect correction is performed as follows. Firstly, some of electrons that are scattered by the scatterer placed on this scattering membrane mask are selectively allowed to pass through an annular opening which is set in a limiting aperture section disposed at the position or in the vicinity of the crossover, and then these scattered electrons allowed to pass are defocused to about the back-scattering range by spherical and chromatic aberrations of an object lens and used as a correction beam to irradiate the wafer. In contrast with a conventional GHOST method wherein the weak correction exposure is performed separately from the primary exposure, with the beam formed by defocusing the inverse pattern of the pattern intended for the primary exposure over the back-scattering range, this technique is characterized in that the proximity effect correction is achieved by carrying out the correction exposure simultaneously with the pattern exposure. Such proximity effect correction as can be implemented by performing the correction exposure simultaneously with the pattern exposure may well contribute to improvement in a throughput. The method of proximity effect correction of this sort has been already reported by G. P. Watson et al. in J. Vac. Sci. Technol., B 13(6), pp.2504-2507 (1995).
As against this, as a mask (referred to as a xe2x80x9cstencil maskxe2x80x9d, hereinafter) utilized in the cell projection lithography or employed in an apparatus (a cell projection lithography type electron-beam exposure system) for this method, there is generally used a mask in which an opening pattern is formed in a substrate that does not allow the electron beam to transmit, for instance, a silicon substrate with a thickness of not less than 20 xcexcm for 50 keV electron beam.
Accompanied with achievement of higher integration of a semiconductor device, as the miniaturization of patterns proceeds, however, a stencil mask made of a thick substrate as described above has started to have the following problems. Namely, when manufacturing a mask, it is difficult to form an opening pattern in a silicon substrate as thick as 20 xcexcm with accuracy so that the variation in size is produced. Further, in respect of the electron-beam exposure, because the mask absorbs the electron-beam and generates heat, problems of lowered durability and variation of the mask position through thermal expansion may arise. Moreover, since there is a requirement to increase the accelerating voltage still further so as to reduce the aberration of the electron optical system and improve the resolution, the mask substrate tends to become thicker causing the above problems more significant.
With the stencil mask, if the mask substrate is made thinner, although the linewidth accuracy of the opening pattern is raised and the amount of heat generation is lowered, the electron beam which should be blocked in the first instance may be allowed to transmit the mask substrate section (non-opening section). As a result, a region of the resist on the wafer that should not be exposed may be exposed, which leads to a poor contrast and a low resolution.
Accordingly, with the object of solving these problems of the stencil mask in the cell projection lithography, in Japanese Patent Application Laid-open No. 97055/1998, there is disclosed a mask for electron-beam exposure, wherein an opening pattern is formed in a relatively thin mask substrate, and, in addition, an electron-beam scattering layer is formed on the backside of the mask for the purpose of scattering the electron-beam that has transmitted said mask. This electron-beam scattering layer may be a layer made of polycrystal such as polysilicon, tungsten silicide, molybdenum silicide, titanium silicide or the like or a uneven layer. It is described, therein, through formation of such an electron-beam scattering layer, the electrons that may transmit the pattern layer of the mask (the non-opening section of the substrate) can be successfully prevented from entering into the wafer.
Further, Japanese Patent Application Laid-open No. 163371/1994 discloses an electron-beam writing apparatus in which an opening is set in a substrate having a thickness less than an electron penetration depth to provide an electron-beam shaping aperture that is used as a mask and further, on the downstream side from the above-mentioned shaping aperture in the electron optical system, a mechanism is set to cut off scattered electrons which have transmitted the substrate section of the shaping aperture (mask). In this invention, there is provided a mechanism to cut off the scattered electrons which have transmitted the substrate section of the shaping aperture by setting a limiting diaphragm of a small diameter in a crossover plane so that only electrons which have passed through the opening section of the mask may be allowed to pass through, while the scattered electrons by the mask substrate section are removed by the limiting diaphragm plate. In addition, another cutting-off mechanism is described therein. Namely, an energy filter is set, and thereby decelerated electrons which have lost a part of their energy by penetrating the mask substrate section are deflected further and then removed by the limiting diaphragm.
As described above, some of stencil masks used in the cell projection lithography, by becoming thinner, may produce scattered electrons. Irradiation of the wafer with these scattered electrons causes a reduction of contrast and a lowering of the resolution so that, in cell projection lithography, it is necessary to provide a means to prevent the scattered electrons from reaching the wafer.
Further, stencil masks are, so far, used solely in the cell projection lithography and no case in which a stencil mask is used in SAL type electron beam exposure method has been reported other than the one by the present inventor.
Now, the proximity effect correction in the SAL type electron-beam exposure method has, hitherto, the following problems.
In general, the correction exposure for one shot of the pattern exposure is always made with a considerably large uniform dose (back-scattering coefficientxcex7) throughout, independent of the pattern density distribution. This brings about a decrease in contrast, resulting in a poor resolution and a small exposure dose margin.
The dose of the correction exposure is normally adjusted by a diameter and a width of an annular opening formed in a limiting aperture section. As this control of the dose of the correction exposure is designed for the whole one pattern, it is impossible, within one pattern region, to adjust the dose of correction exposure to the distribution of the pattern density.
Further, in some wafers, the extent of the proximity effect varies with the underlying pattern. Especially when an underlying pattern of an interconnection or the like is formed of a heavy metal such as tungsten or the like in an underlying layer under the resist layer that is placed on the wafer surface, the image-forming electrons are reflected or back-scattered by that underlying pattern and, consequently, the extents of the proximity effect may become different between the resist region over the region where no underlying pattern is formed and the resist region over the underlying pattern formation region. Even in such a case, hitherto, the dose of the correction exposure cannot be changed within one pattern region so that the adjustment of the dose, according to the underlying pattern, cannot be made.
Meanwhile, in Japanese Patent Application Laid-open No. 274841/1998, the present assignee identified a problem in the proximity effect that, in full pattern writing type electron-beam exposure method or in cell projection lithography, the densities of opening patterns in a stencil mask are different between the central section and the edge section of the cell arrays (because no pattern is formed in the vicinity of the edge section of the cell array) so that the dimension of the patterns transcribed onto a resist varies, and disclosed a mask characterized in that the thickness of the mask substrate is varied with the position so as to change the amount of the electron beam transmitting the mask substrate according to the density of the opening patterns in a stencil mask and an electron-beam exposure method therewith. Further, in a paragraph (0029) in that publication, it is described, xe2x80x9cThrough a change in the thickness of the mask substrate according to the density of the opening patterns in a stencil mask, the amount of electron beam transmitting the non-opening section can be controlled. Namely, in a mask part corresponding to the array central section where the opening patterns are dense, the amount of transmitting electron beam can be reduced, while in a mask part corresponding to the array edge section where the opening patterns are sparse, the amount of transmitting electron beam can be raised. Accordingly, it is possible to attain the identical pattern dimension for the array central section and the array edge section and correct the adverse influence of the proximity effect satisfactorily. It is further described, xe2x80x9cSince an exposure of the whole pattern can be made with one shot, this method can improve a throughput.xe2x80x9d
As seen clearly in this description, the invention disclosed in the above publication is made to correct the proximity effect merely through differences in the rate of the electron transmission, under the assumption that the electrons transmitting non-opening sections are all non-scattered electrons travelling straight on, neglecting any scattered electrons. In short, in this method, the exposure is to be made with the inverse patterns as they are, for which the amount of the transmitting electron beam varies with the pattern density of the original patterns.
This method altogether differs from the so-called GHOST method in which the proximity effect is corrected by performing the weak correction exposure with the beam that is formed by defocusing the inverse pattern of the primary pattern (the positive pattern) over the back-scattering range. In the method described in the above publication, in order to correct the proximity effect, the exposure of the inverse pattern is performed simultaneously with the positive pattern, the inverse pattern without being defocused, so that the image contrast becomes considerably low, which indicates that this method is not suited for the formation of fine patterns. Moreover, because the amount of the transmitting electron beam for the inverse patterns varies with the pattern density of the primary patterns, the contrast also varies with the pattern density and, thus, the linewidth accuracy becomes low.
An object of the present invention is, in the SAL type electron-beam exposure method wherein the proximity effect correction by the GHOST method is simultaneously carried out during the pattern exposure, to adjust the correction dose to the pattern density, and thereby to increase the contrast, improve the resolution and enlarge the dose margin. Another object of the present invention is to improve the linewidth accuracy of the pattern through the adjustment of the correction dose to the back-scattering to which the underlying pattern contributes. A further object of the present invention is to provide a mask for electron-beam exposure and an electron-beam exposure system used in this method.
In accordance with the first aspect of the present invention, there is presented a scattering-angle limiting type electron-beam exposure method in which a mask having a scattering region is used, and a limiting aperture is set to control the amount of scattered electrons that are scattered by said mask to pass through, whereby a scattering contrast is formed from differences in the scattering angles of electrons within the electron beam having passed through said mask, and thereby pattern exposure is performed; wherein
by changing the thickness of the scattering region of the mask according to the pattern density, the scattering angles of the scatter ed electrons are controlled and the amount of the scattered electrons to pass through the limiting aperture is adjusted, and, using these scattered electrons after passing through said limiting aperture for the correction exposure, the proximity effect correction is simultaneously performed during the pattern exposure.
In accordance with the second aspect of the present invention, there is presented the electron-beam exposure method as described as the first aspect, wherein said mask has a structure in which a scattering region made of an electron-beam scatterer with a prescribed pattern is formed over an electron-beam transmittable thin film.
In accordance with the third aspect of the present invention, there is presented the electron-beam exposure method as described as the first aspect, wherein said mask is a mask in which an opening pattern is formed by setting an opening section in the substrate, and said substrate has a scattering region thinner than the electron penetration depth, and said scattering region, having said opening pattern formed therein, includes a region corresponding to a back-scattering range in the wafer.
In accordance with the fourth aspect of the present invention, there is presented the electron-beam exposure method as described as the third aspect, wherein said mask is one of a set of complementary masks that form a prescribed pattern by combining a plurality of mask patterns;
the scattering angles of the scattered electrons are controlled by-setting the thickness of the scattering region of each complementary mask in such a way that the total correction dose for a plurality of exposures which are performed, using a set of these complementary masks, as many times as the number of masks required to form a prescribed pattern, equals to the correction dose when a prescribed pattern is formed using only one mask and through only one exposure; and
simultaneously with forming a prescribed pattern by carrying out exposures as many times as required, using a set of these complementary masks, the proximity effect correction is performed, using the scattered electrons after passing through the limiting aperture for the correction exposure.
In accordance with the fifth aspect of the present invention, there is presented a scattering-angle limiting type electron-beam exposure method in which a mask having a scattering region is used, and a limiting aperture is set to control the amount of scattered electrons that are scattered by said mask to pass through, whereby a scattering contrast is formed from differences in the scattering angles of electrons within the electron beam having passed through said mask, and thereby pattern exposure is performed; wherein
said mask has a structure in which a scattering region made of an electron-beam scatterer with a prescribed pattern is formed over an electron-beam transmittable thin film; and
by changing the thickness of the electron-beam scatterer of the mask according to the back-scattering to which the underlying pattern of the wafer contributes, the scattering angles of the scattered electrons are controlled and the amount of the scattered electrons to pass through the limiting aperture is adjusted, and, using these scattered electrons after passing through said limiting aperture for the correction exposure, the proximity effect correction is simultaneously performed during the pattern exposure.
In accordance with the sixth aspect of the present invention, there is presented the electron-beam exposure method as described as the first aspect, wherein, in the scattering region of the mask that corresponds to the underlying pattern of the wafer and the back-scattering range to which said underlying pattern contributes, the thickness of the scattering region of the mask is set, while taking the back-scattering to which the underlying pattern contributes into account, whereby the scattering angles of the scattered electrons are controlled and the amount of the scattered electrons to pass through the limiting aperture is adjusted, and, using these scattered electrons after passing through said limiting aperture for the correction exposure, the proximity effect correction is simultaneously performed during the pattern exposure.
In accordance with the seventh aspect of the present invention, there is presented the mask for electron-beam exposure that is used in the method described as the first aspect, wherein, in order to control the scattering angle of the scattered electrons, the thickness of the scattering region of the mask is changed in such a way that the proximity effect correction may be performed with correction dose appropriate to the pattern density.
In accordance with the eighth aspect of the present invention, there is presented the mask for electron-beam exposure as described as the seventh aspect, which has a structure in which a scattering region made of an electron-beam scatterer with a prescribed pattern is formed over an electron-beam transmittable thin film.
In accordance with the ninth aspect of the present invention, there is presented the mask for electron-beam exposure as described as the seventh aspect, wherein an opening pattern is formed by setting an opening section in the substrate, and said substrate has a scattering region thinner than the electron penetration depth, and said scattering region, having said opening pattern formed therein, includes a region corresponding to a back-scattering range in the exposed substance.
In accordance with the tenth aspect of the present invention, there is presented a set of complementary masks for the electron-beam exposure that are used in the method described as the fourth aspect, which form a prescribed pattern by combining a plurality of mask patterns; and
the thickness of the scattering region of each complementary mask is set in such a way that the total correction dose for a plurality of exposures which are performed, using a set of these complementary masks, as many times as the number of masks required to form a prescribed pattern, equals to the correction dose when a prescribed pattern is formed using only one mask and through only one exposure.
In accordance with the eleventh aspect of the present invention, there is presented the mask for electron-beam exposure that is used in the method described as the fifth aspect; which has
a structure in which a scattering region made of an electron-beam scatterer with a prescribed pattern is formed over an electron-beam transmittable thin film; wherein
in order to control the scattering angle of the scattered electrons, the thickness of the electron-beam scatterer is changed in such a way that the proximity effect correction may be performed with correction dose appropriate to the back-scattering to which the underlying pattern of the wafer contributes.
In accordance with the twelfth aspect of the present invention, there is presented a scattering-angle limiting type electron-beam exposure system, wherein a mask having a scattering region is used, and a limiting aperture is set to control the amount of scattered electrons that are scattered by said mask to pass through, whereby a scattering contrast is formed from differences in the scattering angles of electrons within the electron beam having passed through said mask, and thereby pattern exposure is performed; wherein
said mask is any mask described as an aspect among the seventh to eleventh aspects; and
said limiting aperture comprises an opening in the central section and a closed zonal opening disposed around said opening in the central section for controlling the amount of scattered electrons to pass through, whereby the correction exposure is provided for the proximity effect correction simultaneously performed during the pattern exposure.
The present invention can adjust the correction dose to the pattern density and thereby increase the contrast, improve the resolution and enlarge the dose margin, in the SAL type electron-beam exposure method wherein the proximity effect correction by the GHOST method is simultaneously carried out during the pattern exposure. Further, in the present invention, through an adjustment of the correction dose to the back-scattering to which the underlying pattern contributes, the linewidth accuracy of the pattern can be improved. Moreover, a mask for electron-beam exposure and an electron-beam exposure system suitable to use in this method can be provided.